The present invention relates to a radiation-curable thermal printing ink and thermal printing ink ribbons which employ such a radiation-curable thermal printing ink for printing character and/or bar code images on articles such as labels. The present invention also relates to methods of making and printing using such radiation-curable thermal printing ink and ink ribbons.
Thermal printing ink ribbons have long been used as a means for printing on various articles information such as bar code images. These thermal printing ink ribbons comprise an ink layer consisting of a colorant and a binder material such as wax on a heat-resistant base, and may be fitted to a printer so that the ink layer is molten under heat of the thermal head and transferred onto package and paper or a label to give a desired thermal bar code image thereon.
In recent years, transferred images that are printed using such a thermal printing ink ribbon have been required to be extremely sharp in appearance. One method of obtaining sharper transferred images includes the addition of fine particles of a heat setting resin such as silicon resin or the like to the ink layer of a thermal transfer ink ribbon whose binder mainly comprises wax. This improves the ability of the ink layer to be separated sharply from the base and results in a sharper image.
Another method includes using a thermally fusible resin such as a thermoplastic resin, instead of wax, as a main constituent of the binder. While the use of a thermoplastic resin is effective to improve heat resistance and wear resistance of transferred images, however, the ability of the ink layer to be sharply transferred from the base is reduced. This fails to produce an image of sufficient clarity. Resin binders and/or waxes of higher melting points can also provide a higher degree of scratch and smear resistance. However, higher print head energies are necessary to achieve the desired flow to promote transfer and adhesion to a receiving article.
There are some limitations on the applications for thermal transfer printing. For example, the properties of the thermal transfer formulations which permit transfer from the carrier to a receiving substrate can place limitations on the permanency of the printer matter. Printed matter from conventional processes can smear or smudge, especially when subjected to a subsequent sorting operation. Additionally, where the surface of a receiving substrate is subject to scratching the problem is compounded. This smearing can make character recognition such as optical character recognition or magnetic ink character recognition difficult and sometimes impossible. In extreme cases, smearing can make it difficult to read bar codes. Additionally, exposure of the image to various chemicals can be detrimental.
U.S. Pat. No. 6,025,017 discloses a UV or visible light curable coating formulation which uses monomers and oligomers for the purpose of reducing or eliminating solvents during the manufacturing process. This coating is cured during the manufacture of the ribbon, prior to any printing, to form a thermoplastic polymer that can be thermally transferred to a receiving substrate.
U.S. Pat. No. 6,040,040 discloses a radiation curable thermal printing ink which is selectively cured during ribbon manufacture, prior to printing. The ink is applied to a substrate in multiple or graded layers.
U.S. Pat. Nos. 5,919,557 and 5,952,098 relate to a thermal transfer medium having reactive components that cross-link when heated. Radiation-curable components are not disclosed.
Many attempts have been made to provide high integrity thermal transfer printing which is resistant to degradation due to chemical, heat and physical damage, some of which are described above. There is a continuing effort to provide alternative thermal transfer media which can form printed images with high resistance to these kinds of damage.
The present invention provides a thermal transfer printer ribbon which comprises a substrate and an ink layer disposed on the substrate, the ink layer having radiation-curable components. The radiation-curable components of the ink layer are compounds such as monomers and/or oligomers which when exposed to radiation cross-link and provide improved resistance to chemical, heat and physical damage to the transferred image. This is accomplished via a thermosetting polymerization mechanism. The ink layer of the present invention can be thermally dried on the ribbon while remaining in the uncured state.
Also included in the present invention is a method of making a thermal transfer printer ribbon in which the ink layer can be thermally dried as a final step in preparation of the ribbon.
The invention also provides a method of thermal transfer printing comprising providing a thermal transfer printer, an ink ribbon having radiation-curable components and a receiving article to be printed. A radiation-curable ink ribbon is positioned between the print head of the printer and the receiving article, and contact is established between the ribbon and the print head. The temperature of selected portions of the ribbon is then elevated to effect transfer of ink to a receiving article. The radiation-curable components are cured after printing, by a variety of methods.
A thermal transfer printer having an actinic energy source attached externally or internally is also included in the present invention.
It is an object of the present invention to provide a thermal transfer medium which provides improved damage resistant images, and permits use of conventional thermal printers.
It is an additional object of the present invention to provide an ink formulation for thermal transfer printing which contains radiation-curable components which can be thermally dried.
It is another object of the invention to provide a ribbon for thermal transfer printing having such a radiation-curable ink.
It is a further object of the invention to provide a method of printing using thermal printers wherein the radiation-curable components are cured after printing.
An additional object of the invention is to provide a thermal transfer printer having an actinic energy source.
An additional object of the invention is to provide greater damage resistant images through the use of thermal transfer printing ink ribbons having ink with radiation-curable components.
These and other objects and advantages of the present invention will become apparent and further understood from the following description with reference to the drawings.